Urea-boosted gas-exfoliation synthesis of lignin-derived porous carbon for zinc ion hybrid supercapacitors

Porous carbons are the favorable candidate cathode for the emerging zinc ion hybrid supercapacitors (ZHSCs). The scale-up production of porous carbon requires green synthesis methods to replace the existing corrosive KOH activation, but most mild activators suffer from low activation efficiency, leading to the inferior surface area and capacitance. Herein, we propose a urea-boosted gas-exfoliation method using K2C2O4 as the mild activator and lignin as the low-cost precursor. Urea has the auxo-action to lower the decomposing temperature of K2C2O4 and to initiate an extra second-step gas-exfoliation. The auxo-action produces the enlarged surface area of porous carbon from 592 m2/g to 1949 m2/g and promotes the morphology regulation from thick carbon bulks to layered carbon sheets. The improved carbon structure contributes to the excellent capacitive performance with a high energy-power density of 126.71 Wh kg−1 at 79.37 W kg−1. Moreover, the energy-storage mechanism is studied by both electrochemical analysis and Density Functional Theory (DFT) method. The DFT calculation demonstrates the effect of interaction between N- and O-containing groups on the adsorption capacity of zinc ions, which can provide information for the structural optimization design of carbon cathode.